Today we made our third dive at Escanaba Trough,
collecting another 30 pushcores to determine the distribution of glass
particles from an eruption on the east side of Central Hill that produced
a roughly 10-square-kilometer lava flow. This is the only young lava flow
in Escanaba Trough, so the glass particles produced when it erupted are
the only ones in the upper foot of the sediment. We have now completed
dives on the north, west, and south sides of the inferred eruption site,
with a fourth dive to the east planned for tomorrow. Our fifth and final
dive at this site will investigate the inferred eruption site on Central
Hill and collect sediment and rock samples near the vent.

Today we began our dive on the main lava flow and worked southward,
traversing back and forth across the contact between sediment and lava.
The edges of the flow were not located exactly where we thought they would
be, as explained in the contribution below from Stephanie on sea-bttom
navigation. With our new observations, which located the actual contact,
we will be able to make some adjustments to the previously collected sidescan
imagery, so that it more accurately shows the location of the flow.

Holes in the surface of an ancient lava lake such as this one (now partially covered with sediment) indicate that the lake first formed a hard crust and then partially drained as it's upslope lava supply dwindled.

The edges of the lava flow we have been studying are sometimes partially buried in sediment, which can make it hard for us to do precise mapping. But wherever there is a hard bottom, animals such as this flytrap anemone (at left) will find and colonize it.

We crossed several lobes of the lava flow during our dive today. These
sheet and lobate flows appear to have erupted rapidly, forming shallow
lava ponds that subsequently drained as the flows advanced beyond them.
In several places, we observed evidence of such pond drainage, which leaves
behind bath-tub rings of lava on the edges of the former pond.

We also collected three pushcores of areas of sediment on top of the
very flat sheet flows and found that the flows were covered by about four
centimeters of mud. Previous research indicates that this part of the seafloor
accumulates about one centimeter of sediment every 250 to 300 years. This
suggests that the these flows could be 1,000-1,200 years old--quite a bit
older than our initial estimates. If this age turns out to be correct,
then the hydrothermal systems in this area may be relatively long-lived,
since they should have started up at about the same time as the eruption
and intrusion of lava into sills within the sediment.

We have somehow been keeping up with processing all the cores, but have
not really had time for more than a cursory look at the glass fragments
recovered from these cores. However, we already know that limu o Pele bubble
fragments as large as 8 mm across have been recovered four kilometers south
of the inferred vent. Samples from our earlier dives contained smaller
particles at similar distances to the north and west. Thus, it is starting
to look like the main current direction during the eruption may have been
from north to south. When all the samples are fully processed, we will
be able to make maps of the size distributions of particles at various
distances and determine how the particles were dispersed and how high into
the water column they were lifted, as Lionel Wilson explained in yesterday’s
posting.

Left image: Thanks to the tireless work of folks like Rob Zierenberg, we have been able to sieve samples from all of the thirty push cores we collect each day. The initial results of this sieving give us hope that we will be able to see significant differences between the size distributions of limu o Pele in our different transects. Right image: Another person who gives us hope (or at least a reason to get up each morning) is Derek Greenwood, the ship's steward. Derek somehow manages to provide creative, delicious meals from fresh ingredients three times a day, despite being unable to go grocery shopping for weeks at a time.

Although the sediment-covered plains that we have been traversing may
seem dull geologically, their inhabitants have continued to surprise and
amaze us. Some of these animals are familiar but have strange shapes or
living habits. Others are so enigmatic that we can't even tell what phylum
they belong to.

Stephanie writes: One of the things that complicates work at sea is
trying to figure out where we are. We know where the ship is by using GPS
navigation, but the ROV Tiburon is somewhere below us at the end
of a two-mile-long tether, and you can't get satellite positions at the
bottom of the sea!

The most precise seafloor navigation is done by deploying a network
of acoustic beacons called transponders on the ocean floor. In this case,
the submersible has a receiver which times the acoustic waves from the
beacons. Using the velocity of sound in water (about 1500 meters per second,
but it varies a bit with depth, temperature, and salinity), these sound
travel-times are converted to distances. Knowing the distance from the
submersible to at least 3 beacons, you can calculate the vehicle's position
by trilateration. (For each beacon, draw a circle centered at the beacon
with a radius equal to the distance to the vehicle. The intersection of
the circles is the location of the vehicle). This is the same way seismologists
calculate the location of earthquakes from multiple seismic stations. Many
people mistakenly call this process triangulation, but triangulation involves
three ("tri") angles, like when you're hiking and you find your position
by using your compass to find your angle to three mountain peaks. When
you use three distances instead of three angles, the process is more accurately
called trilateration. When you navigate using transponders on the seafloor,
it's called long-baseline transponder navigation. However, before you can
do this, you must spend several hours or more deploying transponders and
determining their positions.

For ROV Tiburon, we use a quicker navigation method called short
baseline navigation. This inverts the process described above--the transponders
are on the ship instead of on the sea floor. On the seafloor, you can have
baselines that are a few kilometers long. However, short-baseline navigation
is limited by the length of the surface ship. Sound waves from the submersible
to the ship are turned into distances and the submerisble's position is
calculated relative to the ship's position. A second instrument records
the ROV's speed and heading, which is used to refine the position information.

Left image: This is a typical navigation screen for ROV Tiburon, which is shown at lower left, traveling along the blue track line. The red dots show the approximate navigation "fixes" for the position of the ROV over the previous 20 minutes (as determined by short-baseline navigation). The blue dots show the positions of the R/V Western Flyer during the same period (as determined GPS, which is much more accurate). Right image: This sidescan sonar image shows the seafloor south of the volcanic eruption whose plume materials we have been studying this week, with today's track-line shown in red. The light-colored areas are lava flows; the surrounding sediment plains appear dark. The main eruption center is believed to be near the top of the image. We hope our dive on August 30 will help us locate some of the eruptive vents.

Even when the ROV has good navigation, we have another problem -- we
often need to figure out where we are relative to maps, bathymetry, or
sidescan images that had poorer navigation. In 1996, we collected sidescan
sonar data of the area we are presently exploring. This generates something
that looks like a photo of the seafloor using sound waves instead of light.
Unfortunately, during our 1996 survey, we had problems with our sonar transponder
equipment. For most of the survey we only knew the ship's position and
had to estimate how far behind the ship the sidescan sonar instrument was
being towed. With a 3 kilometer cable, such position estimates can be quite
far off!

On today's dive, we drove over a lava flow that shows up in the 1996
sidescan as a very bright sound reflector and mapped its outline, where
it met the surrounding sediment. We could do this because the basalt lava
is very hard, so it sends back most of the sound energy from the sonar.
Sediments, on the other hand, are softer and absorb the energy, so they
show up as dark patches on the image. While we were diving today, we carefully
determined position of the contact between the sediment and the basalt
and found that it was about 100m away from where it had been mapped on
the 1996 survey.